This invention relates to induction heating apparatus. The herein disclosed embodiment of the invention is particularly intended for soldering a semi-conductor chip and a substrate to a lead frame; however, the principles of the invention can be used for many other heating processes such as bonding or welding parts to each other, heat treating localized portions of a workpiece, and melting of metals.
The principles of high frequency induction heating have long been known and a variety of types of induction heating apparatus are available or have been proposed. The most commonly used type of induction heating apparatus comprises an induction coil which produces a magnetic field when it is energized. When a conductive workpiece is placed in this field, electrical currents are induced in the workpiece and cause it to be heated, as shown for example in U.S. Pat. Nos. 2,503,429 (Dixon) and 3,038,055 (Ziegler). The problem with such heating coils is that their lack of a core decreases the inductance of their magnetic circuit and thus the flux density of the magnetic field that they can generate and it prevents them from being able to concentrate a high flux density into a small area.
This problem has been partially solved by induction heaters that use, in conjunction with a coil, a core made of magnetic materials having a high permeability, as shown for example in U.S. Pat. Nos. 1,725,465 (Manson), 1,996,502 (Brown), 3,008,026 (Kennedy), 3,038,055 (Ross), 3,835,282 (Sass et al), and 4,048,458 (Zink). One of the problems with many of these core induction heaters is that they have high heat losses in their core if the field intensity and frequency of the fluctuating magnetic field generated in them is increased sufficiently to rapidly solder metal. For example it is believed that, if a magnetic field was generated in the core disclosed in the Brown patent of sufficient intensity and frequency to rapidly melt solder placed between its poles, the core disclosed in Brown would quickly melt. Cores made of the laminated magnetic materials used in most transformers have very high losses due both to eddy currents and to the resulting skin effect at frequencies above 20 kilocycles. In addition, the conductive nature of the laminates presents a real danger of electrical shock when used in induction heaters which have a large amount of power supplied to their exciting coils.
U.S. Pat. No. 2,785,263 (Van Iperen) discloses an induction heating device which has a core made of ferrite. Because of ferrite's relatively high magnetic permeability and low conductivity, it represents an ideal material for the cores of induction heaters. Unfortunately, the U- or H-shaped cores disclosed in Van Iperen are of equal cross-sectional area thoughout the path running between their pole pieces. As a result, in order to saturate those pole pieces so that they make the maximum possible contribution to the flux density in a workpiece placed between them, it is necessary to saturate substantially the whole core, which is very inefficient, and which at high frequencies results in huge heat losses.